1. Field of the Invention
The present invention relates to a device (hereinafter referred to as a light emitting device) formed by building an EL (electroluminescence) element on a substrate. In particular, the present invention relates to a sealing technique for an EL panel of a light emitting device in which EL elements formed on a substrate are sealed. Note that a module in which an FPC is connected to an EL panel, and an IC (integrated circuit) is directly mounted through the FPC, is referred to as a light emitting device within this specification.
2. Description of the Related Art
Research into light emitting devices having EL elements as light emitting elements has been active in recent years, and in particular, light emitting devices using organic materials as EL materials have received a lot of attention. These types of light emitting devices are referred to as organic EL displays (OELDs) or organic light emitting diodes (OLEDs).
Light emitting devices do not have viewing angle problems because they are self light emitting unlike liquid crystal display devices. Namely, light emitting devices are more suitable for use out of doors than liquid crystal displays, and a variety of usages have been proposed therefor.
EL elements are structured by an EL layer sandwiched between a pair of electrodes, and the EL layer normally has a lamination structure. A typical lamination structure proposed by Tang, et al., of Eastman Kodak Co., has a lamination structure made from a hole transporting layer, a light emitting layer, and an electron transporting layer. This structure has extremely high light emitting efficiency, and nearly all light emitting devices currently undergoing research and development employ this structure.
Further, additional structures may also be used in which a hole injecting layer, a hole transporting layer, a light emitting layer, and an electron transporting layer are laminated on an anode in the stated order; or in which a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injecting layer are laminated on an anode in the stated order, may also be used. An element such as a fluorescing pigment may also be doped into the light emitting layer. Further, the layers may all be formed by films composed of low molecular weight materials, or they may all be formed by films composed of high molecular weight materials.
All layers formed between the anode and the cathode are defined generically as EL layers within this specification. Therefore, layers described above, namely the hole injecting layers, electron injecting layers, hole transporting layers, light emitting layers and electron transporting layers are all included in EL layers.
Note that a light emitting element formed by a cathode, an EL layer, and an anode is referred to as an EL element within this specification, and that there are two types of formation methods: a method of forming EL layers between two types of stripe shape electrodes formed to be mutually orthogonal (simple matrix method), and a method of forming EL layers between pixel electrodes and opposing electrodes arranged in a matrix shape and connected to TFTs (active matrix method).
Among EL elements, those using fluorescent organic compounds in their EL layers are referred to as organic EL elements, and the biggest problem in putting them to practical use is that the lifetime of the elements is insufficient. Further, element deterioration appears as a widening of non-light emitting regions (dark spots) accompanying long light emission time, and the major cause of this deterioration is due to cathode peeling.
The development of dark spots due to causes such as cathode oxidation and peeling often results from oxygen and moisture within the atmosphere. For example, it is possible to operate elements within the atmosphere if an electrode manufactured by a stable metal such as an MgAg composite is used, but the life of the elements is shortened. It is therefore ideal to perform manufacture of elements all at once in a vacuum or within a glove box under an inert gas atmosphere in order to obtain good element properties.
Namely, the sealing technique used becomes crucial in the manufacture of elements possessing sufficient lifetime for practical usage. A method in which elements are covered by a glass substrate under a dry nitrogen or inert gas atmosphere to seal the periphery by a resin is generally employed.
However, the development of dark spots has been observed even on sealing substrates. It is thought that this is due to promotion of a chemical reaction occurring between the electrodes and residual impurities due to a high electric field produced when driving the elements. In other words, matter adsorbed on the surface and matter emitted from the resin used in sealing exists even if the purity of the gas to be introduced is high, and therefore it is difficult to completely eliminate substances such as oxygen and moisture. The method shown below has been devised in view of this problem.
A cross sectional structure of a general EL panel seal is shown in FIG. 16. Reference numeral 1601 denotes a substrate in FIG. 16, reference numeral 1602 denotes an anode, reference numeral 1603 denotes an EL layer, and reference numeral 1604 denotes a cathode. The anode 1602 and the cathode 1604 are each electrically connected to an external power source. An EL element on the substrate 1601 composed of the anode 1602, the EL layer 1603, and the cathode 1604 is then sealed by a sealing substrate 1607, through a sealant 1608.
An absorption agent (also referred to as a water capturing agent) 1606 made from an absorption substance is added here in order to prevent deterioration of the EL element due to oxygen and moisture existing in a space 1609. This is discussed in detail in the following reference. (Reference: Shin Kawami, Takemi Naito, Hiroshi Ohata, Jin Nakata, “Effect of Water Capturing Agents in Enclosing of Organic EL Elements”, The 45th Japan Society of Applied Physics Proceedings, p. 1223 (1998).
Note that as the absorption agent, there may be used physical absorption substances, typically silica gel, synthetic zeolites, and the like, and chemical absorption substances, typically phosphorous pentoxide, calcium chloride, and the like. However, chemical absorption substances take in absorbed moisture as water of crystallization, and there is no re-emission of moisture, and therefore chemical absorption substances such as barium oxide (BaO) are often used.
Further, as for a method of disposing the absorption agent, the generally adopted methods include: a method in which a space (depressions) for disposing thereon the absorption agent is formed in a sealing substrate, and after placing the absorption agent in the space, a film such as Teflon possessing adhesiveness is bonded thereto in order to prevent dispersion of the absorption agent; and a method in which a bag composed of a permeable substance and filled with the absorption agent is bonded to a sealing substrate so that the absorption agent does not disperse into the space 1609. However, there is also a method in which the absorption agent is directly dispersed within the space 1609.
An absorption agent 1606 such as barium oxide is prepared in the space 1609, as shown in FIG. 16.
Note that the absorption agent such as barium oxide is normally a solid in a powdered state, and therefore there are adopted a method of dispersing the absorption agent within the space as it is, or a method of wrapping this in a film made from a high molecular weight material to be bonded to a sealing substrate.
Further, since absorption agents are generally introduced by hand, the operation under an inert gas atmosphere may become more difficult. Further, in a case where it is intended to provide a wrapped absorption agent, the wrapping itself requires much work.
There are also cases in which introduction of the absorption agent is performed within the atmosphere due to the complexities of working under an inert gas atmosphere. However, in this case, obviously the problem with oxygen and moisture being contained within the space 1609 cannot be avoided.